Asynchronous Transfer Mode-Passive Optical Networks (ATM-PONs) are considered a promising solution for fiber-based access networks for end-users in Fiber-To-The-Home (FTTH)/Fiber-To-The-Building (FTTB) environments. ATM-PONs utilize a tree topology where a passive optical splitter/merger provides broadcasting in the downstream direction and merging in the upstream direction. The splitter/merger typically couples to a single Optical Line Termination unit (OLT) in the upstream direction and to multiple Optical Network Termination units (ONTs) in the downstream direction, thus providing the tree topology. The OLT provides the network-side interface of the optical access network, while the ONTs provide the customer-side interface to the optical access network. Because all incoming ATM cells from ONTs are combined into one cell stream en route to the OLT through the optical merger, there may be collisions among upstream (ONT to OLT) cells from different ONTs unless proper preventative mechanisms are employed.
According to ITU-T G.983.1, a grant allocation technique is used to control upstream cell transfer from ONTs. A grant is permission from the OLT for an ONT to send one upstream cell at a specified slot. Grants are conveyed in downstream Physical Layer Operation and Maintenance (PLOAM) cells. With this mechanism, the OLT can have full control over ONTs in the upstream cell transmission and avoid collisions among them once all ONTs are placed at the same logical distance after ranging. The downstream frame format for a standard 155.52/155.52 Mbits/s PON includes 56 ATM cells with two PLOAM cells, a first PLOAM cell having 27 upstream grants and a second PLOAM cell having 26 upstream grants for a total of 53 upstream grants per frame. The upstream frame format includes 53 cells with three (3) overhead bytes per cell.
One method of generating grants in passive optical networks (PONs) is to use a (static) grant table. In this technique, each PLOAM cell sequentially distributes the next 27/26 entries from the grant table downstream to the ONTs. As would be understood, the upstream bandwidth afforded each table entry can be calculated as: (Total Upstream Bandwidth)/(Table Size). Other methods of grant generation also exist, such as, software control and computer logic control techniques. A disadvantage of software controlled grant generation is the relatively large CPU overhead associated therewith. In addition, computer logic controlled grant generation techniques are considered to be either overly complex to implement, or are considered to provide unsuitably low performance. Accordingly, computer logic control techniques are not yet a viable option for grant generation in PONs. Of the above methodologies, the single grant table approach is the most attractive in that it has arbitrarily fine granularity and requires no constant software overhead. A significant disadvantage, however, is that extremely large sizes of grant tables are necessary to achieve fine granularity. Fine granularity is desirable, for example, to efficiently accommodate PLOAM grants. Accordingly, there is a need to provide a simplified grant generation technique for use in passive optical networks that can also provide fine granularity grants.